I prefer "them" because I've often seen "hir" and such used to refer only to people who have other than he-or-she genders. "They/them" has a long and proud tradition in the English language, and I would like to see it used more.

I, personally, identify very, very strongly as a woman, and I feel like that's being erased when someone refers to me in gender-neutral terms and they know my gender. However, when the gender of the person is totally not known, then I think it's okay. The person's just trying to do the right thing.